Sea Biscuits are large inflated Urchins, and can grow to roughly 8'", so they are not for nano reef tanks! They do burrow so they do best in a deeper sand bed, and they like to collect macro algaes, and rubble on their shells. The standard sea biscuits are very short spined, and a dark brown color. The Red Heart Urchins have longer spines are are a reddish color. Both types look like a sand dollar, but are not as flat.